The invention relates generally to electronic warfare (EW) systems and, more particularly, to devices and techniques for jamming tracking radars.
“Cross-eye” jamming is a known technique for masking an object or platform, e.g., an aircraft, from fire control radars. In a cross-eye system, a portion of the signal transmitted by a fire control radar—the threat signal—is received by the aircraft and is processed to form a jamming signal comprising a pair of inverted amplitude signals that have a phase relationship between them that is 180 degrees out of phase. The jamming signal is then transmitted back toward the radar antenna. When the pair of inverted amplitude signals arrive at the antenna of the fire control radar—the 180 degrees out of phase condition causes wavefront distortion, which further causes the fire control radar to re-align its antenna away from the aircraft. This type of cross-eye system 10 is illustrated in FIG. 1. (It should be noted that a cross-eye system can also be set up to perform as a “power match on transmit” system, where the amplitudes of the jamming signal pair are equal.)
Cross-eye system 10 is mounted within airplane 5. As illustrated in FIG. 1, two antennas (20 and 25) of cross-eye system 10 are placed on opposing wings of airplane 5 such that they are separated by a distance, d, where d is much greater than the associated wavelength, λ, of a fire control radar frequency of interest (i.e., d>>λ). In addition to antennas 20 and 25, cross-eye system 10 also comprises amplifiers 30 and 40, phase shifter 35, and circulators 21 and 26. In operation, each antenna receives the fire control radar signal, or threat signal. The received signal of antenna 20 is provided to amplifier 30, via circulator 21; while the received signal of antenna 25 is provided to amplifier 40, via circulator 26. The gain of amplifiers 30 and 40 are adjusted to produce inverted amplitude output signals. (For example, if antenna 20 receives a higher amplitude signal than antenna 25, then, on transmission, the output signal from amplifier 40 is now proportionally higher than the output signal from amplifier 30.) The output signal from amplifier 40 is provided to antenna 20, via circulator 21, for transmission; while the output signal from amplifier 30 is phased shifted 180° by phase shifter 35 and then provided to antenna 25, via circulator 26, for transmission. The resultant jamming signal has inverted amplitude components that are 180 degrees out of phase with each other. The jamming signal is transmitted in the direction of the fire control radar. When the jamming signal reaches the radar antenna, the resulting wavefront distortion (caused by the inverted amplitude components that are 180 degrees out of phase with each other) causes the fire control radar to attempt to track the jamming signal instead of the aircraft—thus masking the aircraft. The final result is that the jamming signal causes large tracking errors in the fire control radar with the result that any missile launched toward the aircraft is misguided.
In theory, as seen from the description above, cross-eye is a simple technique for providing protection to a platform from weapon systems employing radar guidance. Unfortunately—in practice—the performance of a cross-eye system is critically dependent upon how well the amplitude ratio and the out of phase condition of the jamming signal is controlled—especially in hostile environments where the platform is further subject to severe vibrations and temperature ranges.